Invisibility is commonly dealt with in the books as casting a Veil. For simplicity and in most intents and purposes—in Dresdenverse: Veil = Invisibility.
Also, Veils and Glamours are used very similarly in the Dresden Files canon; therefore, I'd suggest using the Faerie Magic Supernatural Powers as a guideline for building an "Invisibility Power" for Powers bought with Refresh (I'll provide a suggestion of how I'd do so further below, see "Invisible Man—or Woman if You Are One" at the end of the post). I'll assume the cost of the power should be less than -2 or -3, otherwise the PC could just take The Sight and the Channeling, Spirit Element Supernatural powers to get the same abilities (and then some) thereby getting a lot more bang for your buck in Refresh Costs.
That said, the term "Veil" is more encompassing than just "Invisibility" and as such, is a more versatile in your use of power. Personally I'd encourage use of creative spell casting to block from perception, but that's just my opinion. The reason Veils are more versatile is because they can be expanded to block more than just perception based on sight, but can conceivably block perception by other means. For instance, it's not outside the realm of possibility to cast a veil that blocks from sight, sound, smell, touch, and even possibly taste. —Now that's really hiding.
Because I think using Veils as a spell is more useful I'll deal with those details first. I think one of the optional details hidden in the sidebar of the Your Story Rule Book helps the Veil spell behave more consistently with the way Veils are used in the original book series:
Veil Block Exchanges & Duration (Optional Rule p. 255 Sidebar):
“Duration with Evocation veils is largely a
matter of GM judgment call. Personally I wouldn’t go for “1 shift gets
you one extra exchange” with a veil; I’d just let it hang around until
something pierces it, or until the end of the scene. I see them simply
as REALLY GOOD magical Stealth rolls…”
Personal House Rules for Veils I use:
Considering the details above and the review of Blocks I'v recently done, I have come up with my own house rules on Veils, so I figured I'd share them with you. (take 'em or leave 'em, as these are my personal house rulings on the matter of Veils)
Two Types of Veils: A Veil cast as a Spirit Block vs. Perception (Option 1 Below) will last until it is pierced or until the end of a “regular” scene. Once pierced by any opposition the spell is bypassed completely. Thus, if one member of the opposition sees through the veil, the mental stress of attempting to compensate for it over-stresses the energy of the spell and the spell falls apart. Alternately, a Veil cast as a modified Stealth Action using a Spirit Armor effect and Stealth Skill Roll (Option 2 Below) will last until the end of a “regular” scene. Thus, if the veiled subject is noticed by one member of the opposition, he still can remain hidden from others who are not as perceptive.
"Exchange" Judgement Calls: In cases where a scene is drawn out or has other unique situations that could reasonably count as a new exchange for the Veil, then GM should reserve the right to call for a subsequent casting or extension of duration to account for the mental stress of staying Veiled in a prolonged or volatile situation.
Essentially, casting Veils is tricky business, and what counts as an exchange may regularly be based on the circumstances of the situation at hand.
Veil Option 1: Perception Block (via Spirit Block possibly followed by Stealth Roll if Pierced)
This option works by attempting to render the subject invisible. The
advantage of this form of Veil is that its maximum power is fully
realized; however, once the Veil is pierced it the spell is undone,
and the subject is left to their own skill at Stealth to avoid
Game Mechanic: Evocation Power Shifts = Perception Block. Once the Veil is Pierced (Block is Bypassed) a Stealth Skill Roll may be
allowed to avoid detection if the situation allows for it
Variations: Extra shifts of power could be dedicated to blocking additional senses. Perhaps 1 shift extra for each sense you want the block to be effective against, thus a Veil blocking from all five senses would cost 4 extra power.
Veil Option 2: Stealth Action (via Spirit Armor and Stealth Skill Roll)
This option works by lessening the attention the subject gets,
blurring the individual and blending them with their surroundings,
while sending out a subtle mental influence to ignore the subject. The
advantage of this form of Veil is that it isn’t wiped away when it is
pierced, but the disadvantage is that it works at half strength
(similar to an Armor Bonus) until the end of the duration of the spell.
In this situation, the Veiled individual will benefit from the Veil
and use of their Stealth in order to hide. This form of veil and stealth could also require making Stealth
rolls to avoid detection more frequently—just as a regular person hiding
may have to make multiple rolls to avoid perception (perhaps whenever moving).
Game Mechanic: Evocation Power Shifts/2 + Stealth Skill Roll = Stealth Action
Variations: Extra shifts of power could be dedicated to armoring against additional senses. Perhaps 1 shift extra for each sense you want the block to be effective against, thus a Veil Armor bonus from all five senses would cost 4 extra power.
Now, if you're stuck on wanting an Invisibility Supernatural Power by itself here's an example of one that uses the Glamours Faerie Magic Power as a base. Because it only costs -1 it is more limited than the full Glamours power.
Invisible Man—or Woman if You Are One [-1]
Description: You are able to channel your willpower to create a basic veil that hides you from sight.
Skills Affected: Discipline
Minor Invisibility Veil. With a moment of concentration, you may draw a veil over yourself, hiding from sight only. Use your Discipline
to oppose efforts to discover the veil. If the veil is discovered, it
isn’t necessary pierced—but the discoverer can tell that it’s there
and that it’s wrong.
Game Mechanic: Supplemental Action, Roll Discipline to hide from opposition using Alertness or another applicable perception skill
(perhaps Investigation depending on circumstance) to avoid being
detected. –Remember, this is sight only, so hiding from sight doesn’t
mean you can’t be found. The subtle movements of air around you, your odor, and the
sounds you make are still apparent. Commonly, anything with Echoes of The Beast will likely be able to easily find you as their scent and hearing powers are much sharper than that of a "Vanilla Mortal"
Notes: "or a Woman if You Are One" is in homage to the Colin Hay song: "Beautiful World"
Now, the above power is a Non-Caster friendly power that would allow a PC to become invisible. If you want a stronger version of it, make the cost -2 Refresh and add a +2 to the opposed Discipline roll. The build of the above power is just adding the "Hiding" trapping from the Stealth Skill to the Conviction skill with the added flair of "Being Invisible powered by Willpower". Otherwise, hide as normal just use Conviction instead of Stealth for the Skill Roll. Don't bother with rolling blocks, just a roll to hide as normal using Conviction instead of Stealth.
Supporting Block Notes: “1 shift of power adds 1 to the block strength of the block action. Any attack that bypasses the block cancels it out. Optionally, instead of block strength, you can opt to have the effect work as Armor. If you choose the Armor effect, the armor rating is equal to half (rounded down) the shifts put into the spell. The advantage to doing this is that the Armor effect only ends when the spell duration ends—the armor survives a bypassing attack.” p. 252
Block Examples: Standard Block: Three shifts of power create a block strength of Good (+3). Any attack that bypasses the block cancels it out. Thus an attack of Great (+4) in the above case, would break the shield (cancel the block), and the defender would be allowed their standard Defense Skill Roll (Athletics) to avoid the attack.
Armor and Defense Skill: Two shifts of power create an Armor strength of Average (+1). Any attack against the target grants the defender a standard defense roll, with the Armor Reducing Damage if hit. Thus an attack of Great (+4) vs. a Defense Roll of Good (+3) would result in the Defender being hit for 1 Stress; however, the +1 Armor would reduce the stress to nothing, and the Armor bonus would continue to operate until the end of the spell duration.